Infection by mouse mammary tumor virus (MMTV) results in increased incidence of mammary tumors among inbred strains of mice. These virus-specific genes are transmitted either as a milk-bone particle to the newborn or as a proviral sequence integrated into the germ line of the host. The oncogenic potential of these viruses is known to vary, but the exact mechanism for this variation has yet to be determined. This proposal deals with two aspects of virus gene expression and mammary carcinogenesis. Recent data suggests that repression of endogenous proviruses may result from methylation of these sequences. This process is investigated by examining the relationship between transcriptional activity as measured by DNasel sensitivity and level of methylation. Then in vitro modification and transfection procedures are used to assess those specific methylation events essential to repression. In addition, the possibility that altered methylation of endogenous virus-specific sequences is required for transformation in milk-borne virus-free mouse strains is examined after both hormonal and chemical carcinogenic induction of mammary tumors. Finally, since among inbred strains of mice both the endogenous MMTV genotype and the mouse's response to MMTV infection varies, virus-specific RNA and protein levels are studied in recombinant and standard inbred mouse strains to evaluate the activity of specific endogenous viral elements as well as cis and trans acting host regulatory loci. The objective of these studies is to understand the mechanism of tumor induction by genetically transmitted mammary tumor viruses. This is of particular importance in relationship to human disease, since the well established familial pattern would suggest a genetic component. Biochemical examination using virus-specific molecular probes and restriction endonucleases which distinguish various viral genetic elements can be applied to genetically defined phenomenon in the mouse. This approach produces data essential to defining of those factors generally necessary for derepression of normally repressed viral oncogenes in mammalian cells.